This may explain why the north and the south side of the monument are no longer in horizontal alignment.

Rossella Rea, the director of the Colossum and an archaeologist with nearly 30 years' experience, said: "If our doubts are confirmed, we are dealing with two structurally different monuments and at that point it would be necessary to reunite them."

She has now raised the prospect that the same sort of restoration work that saved the Leaning Tower of Pisa more than a decade ago will be needed.

The decorative 12th century tower was closed to the public for 10 years after concerns about its stability and reopened in 2001 after successful reconstruction efforts to straighten its incline and strengthen its foundations.

"Like the Leaning Tower of Pisa, we need to evaluate the best approach but any discussion now is premature," Ms Rea said.

The need for a more comprehensive restoration of the Colosseum, which was built by Emperor Vespasian in 72AD, has been evident in the last couple of years as chunks of masonry and stone have fallen from the arena.

A controversial £21 million proposal by wealthy entrepreneur, Diego Della Valle, head of the Tod's shoe empire, to restore the monument's façade and underground area has also been bogged down by bureaucratic infighting.

The two-year restoration was due to begin in March and the proposal will be presented to Italy's powerful culture ministry tomorrow.